The present invention relates to a method for manufacturing a shutter for a magnetic disk cartridge, and more particularly to a method for molding a plastic shutter from synthetic resin.
Disk-shaped recording media such as a magnetic disk, an optical disk, and a photo-magnetic disk are accommodated in cartridges relatively high in rigidity so that they can be handled with ease and are protected from being adversely affected by dust. In recording data on or reproducing data from a disk-shaped recording medium, the latter is loaded in a data recording and reproducing apparatus while being retained within the cartridge.
One example of a disk cartridge of this type is a 3.5 inch micro floppy disk. The micro floppy disk is composed of an upper half shell and a lower half shell which are made of ABS resin or the like, the shells being relatively high in hardness and rectangular, and a magnetic disk accommodated in the space defined by the upper half shell and the lower half shell. The magnetic disk is a flexible magnetic recording medium manufactured by uniformly forming a magnetic layer on the surface of a disk-shaped macromolecular film support. The magnetic disk has an opening at the center, to which a ring-shaped center core is secured.
Furthermore, as shown in FIG. 2, the disk cartridge 21 has a magnetic head inserting opening 28 formed in the upper and lower half-shells 23 and 24, respectively through which a magnetic head and head pad are insert to write data on the disk or read data from the disk. In order to prevent the entrance of dust into the cartridge through the magnetic head inserting opening 28, that is, in order to prevent the dust from being accumulated on the magnetic disk, the cartridge has provisions for opening and closing the magnetic head inserting opening 28, namely, it is provided with a slide shutter 22.
The shutter 22 is made up of an upper plate 22a, a lower plate 22b, and a side plate 22c through which the upper plate 22a is connected with the lower plate 22b, and thus the shutter 22 is substantially U-shaped in section. The shutter 22 has protrusions 50 which extends from the base of the lower plate 22. The protrusions 50 are slidably engaged with a guide groove (not shown) formed in the front edge portion of the lower half shell 24. The shutter also has another protrusion 51, which extends from the side plate 22c, for engaging with one of the end portions of a torsion spring 52. That is, with the protrusion 50 engaged with the guide groove, the shutter may be slid to open and close the magnetic head inserting opening 28. In this case, the shutter 22 is allowed to slide in a slide region 26 which extends from the periphery of the magnetic head inserting opening 28 to one side of the disk cartridge 21.
Conventionally, the slide shutter has been made of a metal plate such as a thin stainless steel plate. On the other hand, recently a slide shutter molded form synthetic resin has been proposed in the art as disclosed by Japanese Unexamined Published Patent Application No. 231985/1985. Manufacture of the metal slide shutter requires a relatively large number of manufacturing steps and high accuracy. On the other hand, the resin slide shutter can be formed by injection molding. That is, the resin slide shutter can be more easily manufactured, and moreover is lower in manufacturing cost.
A metal mold for molding the above-described resin slide shutter from synthetic resin is as shown in FIG. 3. Slide cores 31 and 32 are provided on both sides of a central core 30. FIG. 3 is an enlarged sectional view of the metal mold, corresponding to a sectional view taken along line A--A in FIG. 2.
The central core 30 and the slide cores 31 and 32 define an injection cavity 40 similar in configuration to the shutter 22. The slide cores 31 and 32 have protrusions 31a and 32a to form the openings 25 in the upper and lower plates 22a and 22b, respectively.
Molten resin is injected into the injecting cavity 40 through a gate (not shown). After the resin thus injected has been solidified to a predetermined degree, the slide cores 31 and 32 are slid (in the directions of the arrows B and C) so as to be moved away from the central core 30. Thereafter, the shutter 22 is ejected from the metal mold with an ejector pin (not shown) provided in the central core 30.
The thickness t of the injecting cavity 40 is extremely small (about 0.2 mm to 0.5 mm). Therefore, if the injection pressure is small, then a so-called "short shot" may occur. This difficulty may be eliminated by increasing the injection pressure. However, in this case, the product may be, for instance, inflated near the gate, or bent unintentionally; that is, it may be deformed. In addition, when the injection pressure is high, the resin is liable to be pushed out of the cavity at the corners of the slide cores 31 and 32 which are held abutted against the central core 30.
This difficulty that the resin is pushed out in this way occurs more frequently after the metal mold has been used many times, so that burrs are formed along the outer edges of the shutter 22 and the edges of the openings 25. The burrs formed on the slide surface of the shutter 22 tend to be fall off as the shutter 22 slides along the slide region 26. Of those burrs, especially the ones formed along the edges of the openings 25 may drop into the cartridge 21 through the openings 28. The burrs once inside the cartridge 21 can scratch the magnetic disk and damage the magnetic disk cartridge itself to the extent that the latter is no longer usable.
Heretofore, selection of an injection molding machine to be used for formation of the shutter 22 depends greatly on the experience of the operator. That is, a standard for manufacture of the shutter with high efficiency has not been clearly established yet.